In the field of digital communications (i.e., transmission and reception), various methods and apparatus are known for the reliable recovery of symbol streams from received signals, depending on the particular transmission system and channel. Generally, such methods and apparatus operate by: (a) analog processing the signal by an input network including RF tuning circuits and an intermediate frequency (IF) processor; (b) analog-to-digital converting the analog processed signal into a sequence of digital samples; (c) demodulating the received digital sequence into a baseband un-equalized symbol stream; (d) equalizing the symbol stream in such a way that the symbols can reliably be mapped to particular points in a so-called symbol constellation. The equalized symbols are then decoded into bit groups, for example, bit pairs, quartets, sextets, octets, and so on, depending upon the complexity of the constellation. Equalization is necessary when the transmission channel and system introduce linear distortions in the signal, resulting in intersymbol interference (ISI), like for example, multipath propagation in the terrestrial broadcast channel; and (e) further data processing which may include Forward Error Correction (FEC) decoding and deinterleaving, among others.
In such methods and apparatus, the equalization process itself is typically adaptive. That is, the un-equalized symbol stream is input to a device or system which monitors its output symbol stream, and adapts its own transfer function to fit the points of its output symbol stream as closely as possible to points of the symbol constellation. Equalization is frequently conducted with the aid of a so-called Decision Feedback Equalizer (DFE), consisting of a Feed Forward Filter and a Feed Back Filter (FFF and FBF, respectively). See John G. Proakis, “Digital Communications”, McGraw-Hill, 2nd Edition, 1989, New York. In many circumstances, the adaptive equalization process is conducted in at least two phases, or operating modes: (a) initialization or convergence phase; and (b) tracking phase. In the initialization or convergence phase, conducted at startup of the equipment, or for example, when an HDTV receiver is tuned to another channel, among other situations, the equalizer employs one or more algorithms, which can be training-based (which use a training sequence as a reference) or blind (without the use of a training sequence). In the initialization phase, the equalizer attempts to reliably initially converge its output symbol stream within an arbitrarily close range of the points on the symbol constellation. An example of a blind convergence algorithm is Godard's Constant Modulus Algorithm (CMA). See D. N. Godard, “Self-Recovering Equalization and Carrier Tracking in Two Dimensional Data Communication Systems”, IEEE Transactions on Communications, Vol. COM-28, pp.1867–1875, November 1980. See also D. N. Godard, U.S. Pat. No. 4,309,770. After initial convergence, the equalization process enters the tracking phase, in which the equalizer transfer function is continuously adapted using an algorithm such as a decision-directed algorithm to keep the decoded symbols within some arbitrarily close range of the points on the symbol constellation. Methods and apparatus of these types are well-known.